Mixed Positive Electrode Active Material, Positive Electrode Comprising Same, And Secondary Battery

ABSTRACT

Provided is a mixed positive electrode active material comprising a large-grain positive electrode active material with an average diameter of 10 μm or greater and a small-grain positive electrode active material with an average diameter of 5 μm or smaller, in which the large-grain positive electrode active material and the small-grain positive electrode active material are coated with different materials between a lithium boron oxide-based composition and metal oxide, respectively.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.15/517,641, filed on Apr. 7, 2017, which is a national phase entry under35 U.S.C. § 371 of International Application No. PCT/KR2015/014041,filed on Dec. 21, 2015, which claims priority to Korean PatentApplication No. 10-2014-0184875 filed on Dec. 19, 2014, the disclosuresof which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a mixed positive electrode activematerial, a positive electrode comprising the same, and a secondarybattery, and more particularly, to a mixed positive electrode activematerial of different materials, a positive electrode comprising thesame, and a secondary battery.

BACKGROUND ART

Lithium secondary battery, recently used in an increased amount, mainlyuses Li-containing cobalt oxide (LiCoO₂) as a positive electrode activematerial. Additionally, the use of Li-containing manganese oxide, suchas, LiMnO₂ with layer crystal structure, and LiMn₂O₄ with a spinelcrystal structure, and a Li-containing nickel oxide LiNiO₂, is alsoconsidered.

The Li-containing cobalt oxide (LiCoO₂) of the positive electrode activematerials is currently widely used because of excellent overallproperties such as superior cycle characteristic, and so on. However,the Li-containing cobalt oxide has several problems such as relativelyhigh price, a low charging and discharging current amount, which isabout 150 mAh/g, unstable crystal structure at 4.3 V of voltage orhigher, risk of fire from reaction with an electrolyte, and so on.

In order to solve the problems, suggestions have been made, whichinclude the technology for coating an outer surface of the Li-containingcobalt oxide (LiCoO₂) with a metal (e.g., aluminum) so as to allowoperation at a high voltage, technology for thermally treatingLi-containing cobalt oxide (LiCoO₂) or mixing with another material, andso on. However, a secondary battery composed of such positive materialmay show weak stability at a high voltage or may have limitedapplication to a mass production process.

Because the lithium manganese oxide such as LiMnO₂ or LiMn₂O₄ hasadvantages of using the eco-friendly manganese which is plentiful as araw material, it gathers many attentions as a positive electrode activematerial that can replace LiCoO₂, but the lithium manganese oxide hasdisadvantages such as small capacity and bad cycle characteristic.

The lithium nickel based oxide such as LiNiO2 costs less than thecobalt-based oxide, while it shows a high discharge capacity whencharged at 4.3 V. Accordingly, a reversible capacity of the doped LiNiO₂may approach to about 200 mAh/g which exceeds a capacity of LiCoO₂(about 165 mAh/g). However, the LiNiO₂-based oxide has problems of rapidphase transition of a crystal structure according to a volume changeaccompanied with charge/discharge cycle, and generating of an excessgases during cycle.

In order to solve the above problem, there is suggested a lithiumtransition metal oxide, which is in a form in which a portion of nickelis substituted with another transition metal such as manganese, cobalt,and so on. The nickel-based lithium transition metal oxide substitutedwith metal has advantages of relatively excellent cycle characteristicand capacity characteristic. However, the cycle characteristic israpidly lowered when used for a long time, and stability problemoccurring from storing at a high temperature is not yet solved.

Further, as the recent mobile device gradually becomes high-functionedto provide various functions while being light-weighted and miniaturizedcontinuously, and as attentions are received on the secondary battery asa power source of an electrical vehicle EV and a hybrid electricalvehicle HEV, which are suggested as methods for solving the airpollution of a gasoline vehicle and a diesel vehicle which use fossilfuel, use of the secondary battery is expected to further increase.Given this, attentions are growing for not only the above problems, butalso the problems of battery stability and high-temperature storingcharacteristic at a state of high level capacity and high electricalpotential.

Accordingly, a new technology for simultaneously solving problems ofoutput and service life characteristics is highly requested.

DISCLOSURE Technical Problem

The present disclosure is designed to solve the problems of the relatedart, and therefore, the present disclosure is directed to providing amixed positive electrode active material simultaneously satisfying bothoutput characteristic at a high voltage and service life characteristicat a high temperature, a positive electrode comprising the same, and asecondary battery.

The other objectives and advantages of the present disclosure can beunderstood with the following description and more clearly with theembodiments of the present disclosure. Also, it will be easilyunderstood that the objects and advantages of the present disclosure maybe realized by the means shown in the appended claims and combinationsthereof.

Technical Solution

An aspect of the present disclosure provides a mixed positive electrodeactive material according to the following embodiments.

A first embodiment relates to a mixed positive electrode active materialincluding a large-grain positive electrode active material having anaverage diameter of 10 μm or greater and a small-grain positiveelectrode active material having an average diameter of 5 μm or less,and a coating layer disposed on a surface of each of the large-grainpositive electrode active material and the small-grain positiveelectrode active material, wherein the coating layers include differentmaterials, wherein one of the materials is a lithium boron oxide-basedcomposition and the other of the materials is a metal oxide.

In the first embodiment, a second embodiment relates to the mixedpositive electrode active material, wherein the lithium boronoxide-based composition includes one of lithium metaborate, lithiumtriborate, lithium tetraborate, lithium pentaborate, lithium heptaborateand lithium octaborate, or mixtures of at least two of them.

In the first or second embodiment, a third embodiment relates to themixed positive electrode active material, wherein the lithium boronoxide-based composition includes one of LiBO₂, LiBO₃, LiB₃O₅, Li₂B₃O₇,Li₂B₄O, Li₂B₄O₇, Li₂B₅O₈, LiB₇O₁₂, Li₂B₈O₁₃ and Li₁₀B₈O₂₄, or mixturesof at least two of them.

In any one of the first to third embodiments, a fourth embodimentrelates to the mixed positive electrode active material, wherein thelithium boron oxide-based composition is derived from boron (B), a boron(B) compound or combinations thereof.

In the fourth embodiment, a fifth embodiment relates to the mixedpositive electrode active material, wherein the boron (B) compoundincludes boric acid (H₃BO₃).

In the fourth embodiment, a sixth embodiment relates to the mixedpositive electrode active material, wherein the coating layer includingthe lithium boron oxide-based composition is formed by mixing thepositive electrode active material with the boron (B), the boron (B)compound or combination thereof, and performing thermal treatment.

In any one of the first to sixth embodiments, a seventh embodimentrelates to the mixed positive electrode active material, wherein thecoating layer including the metal oxide is formed by mixing the positiveelectrode active material with the metal oxide, and performing thermaltreatment.

In any one of the first to seventh embodiments, an eighth embodimentrelates to the mixed positive electrode active material, wherein thelithium in the coating layer including the lithium boron oxide-basedcomposition is present in an amount of 0.05 to 0.2 parts by weight basedon 100 parts by weight of the total weight of the large-grain positiveelectrode active material or the small-grain positive electrode activematerial.

In any one of the first to eighth embodiments, a ninth embodimentrelates to the mixed positive electrode active material, wherein themetal oxide includes one of magnesium oxide, alumina, niobium oxide,titanium oxide and tungsten oxide, or mixtures of at least two of them.

In any one of the first to ninth embodiments, a tenth embodiment relatesto the mixed positive electrode active material, wherein a weight ratiobetween the large-grain positive electrode active material including thecoating layer and the small-grain positive electrode active materialincluding the coating layer is 5:5 to 8:2.

In any one of the first to tenth embodiments, an eleventh embodimentrelates to the mixed positive electrode active material, wherein atleast one of the large-grain positive electrode active material and thesmall-grain positive electrode active material islithium·nickel·manganese·cobalt complex oxide (NMC).

In the eleventh embodiment, a twelfth embodiment relates to the mixedpositive electrode active material, wherein thelithium·nickel·manganese·cobalt complex oxide isLi_((1+δ))Mn_(x)Ni_(y)Co_((1−x−y−z))M_(z)O₂ (M is at least one elementselected from the group consisting of Ti, Zr, Nb, Mo, W, Al, Si, Ga, Geand Sn, and −0.15<δ<0.15, 0.1≤x≤0.5, 0.6≤x+y+z<1.0, 0≤z≤0.1).

Another aspect of the present disclosure provides a positive electrodeaccording to the following embodiments.

A thirteenth embodiment relates to a positive electrode including anelectrode current collector, and a positive electrode active materiallayer formed on at least one surface of the electrode current collectorand including the mixed positive electrode active material according toany one of the first to twelfth embodiments, a conductor and a binder.

In the thirteenth embodiment, a fourteenth embodiment relates to thepositive electrode, wherein the conductor is at least one selected fromthe group consisting of graphite, carbon black, acetylene black, ketjenblack, channel black, furnace black, lamp black, thermal black, carbonfiber, metal fiber, fluorocarbon, aluminum powder, nickel powder, zincoxide, potassium titanate and titanium oxide, and the binder is at leastone selected from the group consisting of polyvinylidene fluoride,vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP),polyvinyl alcohol, carboxymethyl cellulose (CMC), starch,hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone,polytetrafluoroethylene, polyethylene, polypropylene,ethylene-propylene-diene ter polymer (EPDM), sulfonated EPDM, styrenebutadiene rubber (SBR), fluorine rubber, polyacrylonitrile andpolymethylmethacrylate.

In the thirteenth or fourteenth embodiment, a fifteenth embodimentrelates to the positive electrode, wherein the positive electrode is fora secondary battery.

Still another aspect of the present disclosure provides a method forfabricating a mixed positive electrode active material according to thefollowing embodiments.

A sixteenth embodiment relates to a method for fabricating a mixedpositive electrode active material including (S1) preparing alarge-grain positive electrode active material having an averagediameter of 10 μm or greater and a small-grain positive electrode activematerial having an average diameter of 5 μm or less, (S2) mixing thelarge-grain positive electrode active material or the small-grainpositive electrode active material with boron (B), a boron (B) compoundor combination thereof, and performing thermal treatment to form acoating layer including a lithium boron oxide-based composition on asurface of the large-grain positive electrode active material or thesmall-grain positive electrode active material, (S3) mixing the positiveelectrode active material that has not gone through the (S2) among thelarge-grain positive electrode active material and the small-grainpositive electrode active material with a metal oxide, and performingthermal treatment to form a coating layer including metal oxide on asurface of the positive electrode active material, and (S4) mixing theresults of the (S2) and the (S3), a conductor and a binder.

In the sixteenth embodiment, a seventeenth embodiment relates to themethod for fabricating a mixed positive electrode active material,wherein the thermal treatment in the step of forming the coating layerincluding the lithium boron oxide-based composition is performed between250 and 500° C., and the thermal treatment in the step of forming thecoating layer including the metal oxide is performed between 300 and800° C.

In the sixteenth or seventeenth embodiment, an eighteenth embodimentrelates to the method for fabricating a mixed positive electrode activematerial, wherein the thermal treatment is performed under anatmospheric environment.

In any one of the sixteenth to eighteenth embodiments, a nineteenthembodiment relates to the method for fabricating a mixed positiveelectrode active material, wherein the boron (B), the boron (B) compoundor combination thereof is present in an amount of 0.05 to 0.2 parts byweight based on 100 parts by weight of the total weight of the positiveelectrode active material.

In any one of the sixteenth to nineteenth embodiments, a twentiethembodiment relates to the method for fabricating a mixed positiveelectrode active material, wherein the metal oxide is present in anamount of 0.05 to 0.2 parts by weight based on 100 parts by weight ofthe total weight of the positive electrode active material.

Advantageous Effects

The present disclosure gives the following effects. The presentdisclosure has an advantage of providing a positive electrode havingexcellent rolling density, by mixing two types of positive electrodeactive materials having different average diameters.

Further, the present disclosure has an advantage of providing asecondary battery simultaneously enhanced with output characteristic andhigh-temperature service life characteristic, by using heterogeneouspositive electrode active materials respectively coated with a lithiumboron oxide-based composition and metal oxide.

DESCRIPTION OF DRAWINGS

The accompanying drawings illustrate preferred embodiments of thepresent disclosure and, together with the foregoing disclosure, serve toprovide further understanding of the technical spirit of the presentdisclosure. However, the present disclosure should not be construed asbeing limited to the drawings.

The FIGURE is a graph illustrating recovery capacity and resistancechange of Example 1 and 2, and Comparative Examples 1 and 2.

BEST MODE

Hereinafter, preferred embodiments of the present disclosure will bedescribed in detail with reference to the accompanying drawings. Priorto the description, it should be understood that the terms used in thespecification and the appended claims should not be construed as limitedto general and dictionary meanings, but interpreted based on themeanings and concepts corresponding to technical aspects of the presentdisclosure on the basis of the principle that the inventor is allowed todefine terms appropriately for the best explanation. Therefore, theembodiments disclosed in the present specification and theconfigurations illustrated in the drawings are merely the most preferredembodiments of the present disclosure, and not all of them represent thetechnical ideas of the present disclosure, and thus it should beunderstood that there may be various equivalents and modified examplesthat could substitute therefor at the time of filing the presentapplication.

A mixed positive electrode active material according to an embodimentmay include two types of positive electrode active materials havingdifferent diameters, and preferably, may be a mixture of a large-grainpositive electrode active material having an average diameter of 10 μmor greater, and a small-grain positive electrode active material havingan average diameter of 5 μm or less.

When two types of the positive electrode active materials havingdifferent diameters are mixed, pores between the large-grain positiveelectrode active materials may be filled with the small-grain positiveelectrode active material. Therefore, a current collector may be coatedwith a high rolling density, and ultimately, a positive electrode and abattery having excellent energy density may be fabricated.

Examples of the positive electrode active material applicable in thepresent disclosure are not limited, and thus may include any material aslong as it can allow ion intercalation-deintercalation during charge anddischarge. Preferably, the material may preferably belithium·nickel·manganese·cobalt complex oxide (NMC), or morespecifically, the material may beLi(₁₊₆₇₎Mn_(x)Ni_(y)Co_((1−x−y−z))M₂O₂(M is at least one elementselected from the group consisting of Ti, Zr, Nb, Mo, W, Al, Si, Ga, Geand Sn, and −31 0.15<δ<0.15, 0.1≤x≤0.5, 0.6<x+y+z<1.0, 0≤z≤0.1).

The large-grain positive electrode active material may have an averagediameter of 10 μm or greater, and preferably, from 10 μm and 20 μm. Whenthe average diameter is less than 10 μm, there is a problem in which anelementary particle is difficult to be included due to small pore.

Further, the small-grain positive electrode active material may have anaverage diameter of 5 μm or less, preferably 1 μm to 5 μm, morepreferably 3 μm to 5 μm. In this case, when the average diameter exceeds5 μm, there is a problem in which an elementary particle is difficult tobe included within the pore formed by the large-grain positive electrodeactive material.

The mixed positive electrode active material according to an embodimentof the present disclosure includes the large-grain positive electrodeactive material and the small-grain positive electrode active materialeach coated with different materials, in which the different materialsare a lithium boron oxide (LBO)-based composition and a metal oxide.

Specifically, the mixed positive electrode active material may be thelarge-grain positive electrode active material having a coating layerincluding a lithium boron oxide-based composition-the small-grainpositive electrode active material having a coating layer including ametal oxide, or the small-grain positive electrode active materialhaving a coating layer including a lithium boron oxide-basedcomposition-the large-grain positive electrode active material having acoating layer including a metal oxide, and preferably, the small-grainpositive electrode active material having a coating layer including alithium boron oxide-based composition-the large-grain positive electrodeactive material having a coating layer including a metal oxide.

In a specific embodiment of the present disclosure, the lithium boronoxide-based composition may include one of lithium metaborate, lithiumtriborate, lithium tetraborate, lithium pentaborate, lithium heptaborateand lithium octaborate, or mixtures of at least two of them.

In a specific embodiment of the present disclosure, the lithium boronoxide-based composition may include one of LiBO₂, LiBO₃, LiB₃O₅,Li₂B₃O₇, Li₂B₄O, Li₂B₄O₇, Li₂B₅O₈, LiB₇O₁₂, Li₂B₈O₁₃ and Li₁₀B₈O₂₄, ormixtures of at least two of them.

In a specific embodiment of the present disclosure, the lithium boronoxide-based composition may be derived from boron (B), a boron (B)compound or combinations thereof.

In this instance, the boron (B) compound may include boric acid (H₃BO₃).

In a specific embodiment of the present disclosure, the coating layerincluding the lithium boron oxide-based composition may be formed bymixing the positive electrode active material; with the boron (B), theboron (B) compound or combinations thereof, and performing thermaltreatment.

In this instance, the thermal treatment may be performed between 250 and500° C.

The coating layer including the metal oxide may be formed by mixing thepositive electrode active material with the metal oxide and performingthermal treatment, and in this instance, the thermal treatment may beperformed between 300 and 800° C.

Additionally, the thermal treatment may be performed under anatmospheric environment.

In a specific embodiment of the present disclosure, the coating layercoated on the surface of the positive electrode active material maycover the surface of the positive electrode active material in part orin whole.

In a specific embodiment of the present disclosure, the lithium in thecoating layer including the lithium boron oxide-based composition may bepresent in an amount of 0.05 to 0.2 parts by weight based on 100 partsby weight of the total weight of the large-grain positive electrodeactive material or the small-grain positive electrode active material.

The mixed positive electrode active material according to an embodimentmay be formed in a way in which the large-grain positive electrodeactive material and the small-grain positive electrode active materialare respectively coated with different materials between lithiumtriborate and metal oxide.

Specifically, the mixture may be a mixture of large-grain positiveelectrode active material coated with lithium triborate/small-grainpositive electrode active material coated with metal oxide, or a mixtureof small-grain positive electrode active material coated with lithiumtriborate/large-grain positive electrode active material coated withmetal oxide, and preferably, the mixture of small-grain positiveelectrode active material coated with lithium triborate/large-grainpositive electrode active material coated with metal oxide may beprovided.

In one example, for the metal oxide, any material may be used withoutlimitations as long as it may be coated on the positive electrode activematerial and have excellent stability at a high voltage. Examples mayinclude, without limitation, transition metal oxide such as Nb, Ti, Zn,Sn, Zr, and W, lantanide metal oxide such as Ce, as well as relativelylight metal oxide such as Mg, Al, B, and Si, and preferably, Mg, Al, Nb,Ti, and W.

In a specific embodiment of the present disclosure, the metal oxide mayinclude one of magnesium oxide, alumina, niobium oxide, titanium oxideand tungsten oxide, or mixtures of at least two of them.

The positive electrode active material according to an embodiment mayuse the large-grain positive electrode active material including thecoating layer and the small-grain positive electrode active materialincluding the coating layer mixed at a weight ratio of 5:5 to 8:2, andpreferably, at a weight ratio of 6:4 to 7:3. When the amount of thelarge-grain positive electrode active material is less than 50 wt %, itresults to a small number of the pores formed by the large-grainmaterial, making it difficult to reduce packing density with thesmall-grain material. When the amount of the large-grain positiveelectrode active material exceeds 80 wt %, the effect of improvingoutput by the small-grain material becomes inadequate.

According to another aspect of the present disclosure, theabove-described mixed positive electrode active material is prepared bythe following method:

(S1) preparing a large-grain positive electrode active material havingan average diameter of 10 μm or greater and a small-grain positiveelectrode active material having an average diameter of 5 μm or less;

(S2) mixing the large-grain positive electrode active material or thesmall-grain positive electrode active material with boron (B), a boron(B) compound, or combinations thereof and performing thermal treatmentto form a coating layer including a lithium boron oxide-basedcomposition on the surface of the large-grain positive electrode activematerial or the small-grain positive electrode active material;

(S3) mixing the positive electrode active material that has not gonethrough (S2) among the large-grain positive electrode active materialand the small-grain positive electrode active material with a metaloxide and performing thermal treatment to form a coating layer includingmetal oxide on the surface of the positive electrode active material;and

(S4) mixing the results of (S2) and (S3), a conductor and a binder.

In this instance, for the positive electrode active material, thelithium boron oxide-based composition and the metal oxide used, areference is made to the above description.

In a specific embodiment of the present disclosure, the boron (B), theboron (B) compound, or combinations thereof may be thermally treatedbetween 250 and 500° C., and the metal oxide may be thermally heatedbetween 300 and 800° C.

In a specific embodiment of the present disclosure, the thermaltreatment may be performed under an atmospheric environment.

In a specific embodiment of the present disclosure, the boron (B), theboron (B) compound, or combination thereof may be present in an amountof 0.05 to 0.2 parts by weight based on 100 parts by weight of the totalweight of the positive electrode active material.

In a specific embodiment of the present disclosure, the metal oxide maybe present in an amount of 0.05 to 0.2 parts by weight based on 100parts by weight of the total weight of the positive electrode activematerial.

The positive electrode according to another embodiment may include anelectrode current collector, and a positive electrode active materialformed on at least one surface of the electrode current collector andincluding the mixed positive electrode active material described above,a conductor and a binder.

The electrode current collector may not be limited to any specificmaterial as long as it has conductivity and does not cause a chemicalchange in the related art, and may include, for example, stainlesssteel, aluminum, nickel, titanium, sintered carbon, or, aluminum orstainless steel surface-treated with carbon, nickel, titanium, silver,and so on. Further, the electrode current collector may have micro bumpsformed on the surface thereof to enhance the adhesion strength of thepositive electrode active material, may have various forms such as film,sheet, foil, net, porous body, foam, nonwoven fabric, and so on, and mayhave a thickness of 3 μm to 500 μm.

The conductor may not be limited to any specific example as long as ithas conductivity and does not cause a chemical change in the relatedart, and may use a conductive material such as graphite such as naturalgraphite or artificial graphite; carbon black such as carbon black,acetylene black, ketjen black, channel black, furnace black, lamp black,or thermal black; conductive fiber such as carbon fiber or metal fiber;metal powder such as fluorocarbon, aluminum, or nickel powder;conductive whisker such as zinc oxide or potassium titanate; conductivemetal oxide such as titanium oxide; and polyphenylene derivatives.Preferably, the conductor may be at least one selected from the groupconsisting of graphite, carbon black, acethylene black, ketjen black,channel black, furnace black, lamp black, thermal black, carbon fiber,metal fiber, fluorocarbon, aluminum powder, nickel powder, zinc oxide,potassium titanate, and titanium oxide, and may generally be added in anamount from about 1 wt % to 20 wt % based on the total weight of themixture including the mixed positive electrode active material.

Further, any component may be used as the binder applicable in thepresent disclosure without limitations as long as the componentcontributes to bonding of the mixed positive electrode active materialand the conductor and bonding to the electrode current collector, andmay preferably be at least one selected from the group consisting ofpolyvinylidene fluoride, vinyldene fluoride-hexafluoropropylenecopolymer (PVDF-co-HFP), polyvinyl alcohol, carboxymethyl cellulose(CMC), starch, hydroxypropylcellulose, regenerated cellulose,polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene,polypropylene, ethylene-propylene-diene ter polymer (EPDM), sulfonatedEPDM, styrene butadiene rubber (SBR), fluorine rubber, polyacrylonitrileand polymethylmethacrylate and may generally be added in an amount from1 wt % to 20 wt % based on the total weight of the mixture including thepositive electrode active material.

Further, the positive electrode according to an embodiment mayoptionally include a filler, and the filler applicable in the presentdisclosure may not be specifically limited as long as it is fibrousmaterial that does not cause a chemical change in corresponding battery.Non-limiting examples may include olefin-based polymer such aspolyethylene and polypropylene; and fibrous material such as glass fiberor carbon fiber.

The secondary battery including the positive electrode described aboveaccording to another embodiment may be provided, in which the secondarybattery may preferably be a lithium secondary battery, and the secondarybattery may have a driving voltage of 4.25 or greater.

The lithium secondary battery may be composed of the positive electrodedescribed above, a negative electrode, a separator, and a Li-containingnonaqueous electrolyte, and the other components of the lithiumsecondary battery according to an embodiment excluding the positiveelectrode will be explained below.

The negative electrode may be fabricated by applying and drying anegative electrode material on a negative electrode current collector,and as necessary, the components described above may be furtherincluded.

The negative electrode material may include, for example, carbon such ashard carbon or graphite carbon; metal complex oxide such asLixFe₂O₃(0≤x≤1), Li_(x)WO₂(0≤x≤1), Sn_(x)Me_(1−x), Me′_(y)O_(z) (Me: Mn,Fe, Pb, Ge; Me′: Al, B, P, Si, 1, 2, 3 family elements of the periodictable, halogen; (0<x≤1; 1≤y≤3; 1≤z≤8); lithium metal; lithium alloy;silicon alloy; tin alloy; metal oxide such as SnO, SnO₂, PbO; conductivepolymer such as polyacetylene; and Li—Co—Ni materials.

The negative electrode current collector is generally fabricated to athickness from 3 μm to 500 μm. The negative electrode current collectormay not be limited to any specific material as long as it hasconductivity and does not cause a chemical change in correspondingbattery, and may include, for example, copper, stainless steel,aluminum, nickel, titanium, sintered carbon, copper or stainless steelsurface-treated with carbon, nickel, titanium, silver, and so on,aluminum-cadmium alloy, and so on. Further, likewise the positiveelectrode current collector, the negative electrode current collectormay have increased adhesion by having micro bumps formed on the surfacethereof, and may be used in various forms such as film, sheet, foil,net, porous body, foam, nonwoven fabric, and so on.

The separator may be interposed between the positive electrode and thenegative electrode, and a thin insulating film having high ionpenetration and mechanical strength may be used. A pore diameter of theseparator may be generally from 0.01 μm to 10 μm, and a thickness may begenerally from 5 μm to 300 μm. The separator may be, for example,olefin-based polymer such as polypropylene with the chemical resistanceand the hydrophobic property; and a sheet or nonwoven fabric made fromglass fiber or polyethylene. When the solid electrolyte such as polymeris used for the electrolyte, the solid electrolyte may act also as theseparator.

Li-containing nonaqueous electrolyte may be composed of nonaqueouselectrolyte and lithium salt. For the nonaqueous electrolyte, nonaqueouselectrolytic solvent, solid electrolyte, and inorganic solid electrolytemay be used.

The nonaqueous electrolytic solvent may include, for example, aproticorganic solvent such as N-methyl-2-pyrrolidinone, propylene carbonate,ethylene carbonate, butylene carbonate, dimethyl carbonate, diethylcarbonate, gamma-butyrolactone, 1,2-dimethoxyethane, tetrahydroxyfranc,2-methyl tetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide,dimethylformamide, dioxolane, acetonitrile, nitromethane, form acidmethyl, methyl acetate, phosphoric acid triester, trimethoxy methane,dioxolane derivatives, sulforan, methyl sulforan,1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives,tetrahydrofuran derivatives, ether, pyropionic methyl, and propionicethyl.

The organic solid electrolyte may include, for example, polyethylenederivatives, polyethylene oxide derivatives, polypropylene oxidederivatives, phosphoric acid ester polymer, poly agitation lysine,polyester sulfide, polyvinyl alcohol, polyvinylidene fluoride, and apolymer including ionic dissociable group.

The inorganic solid electrolyte may include, for example, nitride,halide, and sulphate of lithium, such as Li₃N, LiI, Li₅NI₂,Li₃N-LiI-LiOH, LiSiO₄, LiSiO₄-LiI-LiOH, Li₂SiS₃, Li₄SiO₄,Li₄SiO₄-LiI-LiOH, and Li₃PO₄-Li₂S-SiS₂.

The lithium salt is a material that is readily soluble in the nonaqueouselectrolyte described above, and may include, for example, LiCl, LiBr,LiI, LiCO₄, LiBF₄, LiB₁₀Cl₁₀, LiPF₆, LiCF₃SO₃, LiCF₃CO₂, LiAsF₆, LiSbF₆,LiAlCl₄, CH₃SO₃Li, (CF₃SO₃Li, (CF₃SO₂)₂NLi, chloroborane lithium, loweraliphatic carboxylic acid lithium, lithium tetraphenyl borate, andimides.

For the enhancement of the charge/discharge characteristics, flameretardancy, and so on, the nonaqueous electrolyte may be added with, forexample, pyridine, triethylphosphite, triethanolamine, cyclic ether,ethylenediamine, n-glyme, hexaphosphoric triamide, nitrobenzenederivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone,N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammoniumsalt, pyrrole, 2-methoxy ethanol, aluminum trichloride, and so on.According to embodiments, in order to impart flame retardancy, thenonaqueous electrolyte may further contain halogen-containing solventssuch as carbon tetrachloride and ethylene trifluoride. Further, in orderto improve high-temperature storage characteristic, the nonaqueouselectrolyte may further contain carbon dioxide gas, and may furtherinclude fluoro-ethylene carbonate (FEC), propene sultone (PRS),fluoro-propylene carbonate (FPC) and so on.

Hereinafter, for more specified description, the present disclosure willbe described in detail with reference to Examples. However, the Examplesaccording to the present disclosure can be modified in various forms,and the scope of the present disclosure is not to be construed as beinglimited to the Examples described below. The Examples according to thepresent disclosure are provided in order to give more completedescription of the present disclosure to those having average knowledgein the art.

Example 1

100 parts by weight of positive electrode active materialLiMn_(0.2)Ni_(0.6)Co_(0.2)O₂ having an average diameter of 10 μm weremixed with 0.1 parts by weight of alumina (Al₂O₃), and thermally treatedat 500° C. in an atmospheric environment to prepare positive electrodeactive material particles A having a coating layer on the surface of thepositive electrode active material.

Subsequently, 100 parts by weight of positive electrode active materialLiMn_(0.2)Ni_(0.6)Co_(0.2)O₂ having an average diameter of 5 μm weremixed with 0.1 parts by weight of boric acid (H₃BO₃), and thermallytreated at 300° C. in an atmospheric environment to prepare positiveelectrode active material particles B having a coating layer on thesurface of the positive electrode active material.

The mixed positive electrode active material was fabricated by mixingthe positive electrode active material particles A and the positiveelectrode active material particles B at a weight ratio of 70:30, andthen the positive electrode was fabricated by mixing the mixed positiveelectrode active material, the conductor (carbon black), and the binder(polyvinylidene fluoride) respectively in 92.5:3.5:4 of a weight ratioand coating the result on the electrode current collector, after whichthe secondary battery was fabricated by inserting the separator betweenthe positive electrode and the negative electrode, which are fabricatedabove, and sealing with an aluminum pouch case.

Example 2

100 parts by weight of positive electrode active materialLiMno_(0.2)Ni_(0.6)Co_(0.2)O₂ having an average diameter of 10 μm weremixed with 0.1 parts by weight of boric acid (H₃BO₃), and thermallytreated at 300° C. in an atmospheric environment to prepare positiveelectrode active material particles C having a coating layer on thesurface of the positive electrode active material.

Subsequently, 100 parts by weight of positive electrode active material

LiMn_(0.2)Ni_(0.6)Co_(0.2)O₂ having an average diameter of 5 μm weremixed with 0.1 parts by weight of alumina (Al₂O₃), and thermally treatedat 500° C. in an atmospheric environment to prepare positive electrodeactive material particles D having a coating layer on the surface of thepositive electrode active material.

The secondary battery may be fabricated with the same method used inExample 1 except that the mixed positive electrode active material usedherein was fabricated by mixing the positive electrode active materialparticles C and the positive electrode active material particles DF at aweight ratio of 70:30.

Comparative Example 1

The positive electrode active material was fabricated by solely usingthe positive electrode active material particles C, after which thepositive electrode was fabricated by mixing the conductor (carbon black)and the binder (polyvinylidene fluoride) respectively in a weight ratioof 92.5:3.5:4 and coating the result on the electrode current collector.The secondary battery was then fabricated by inserting the separatorbetween the positive electrode and the negative electrode, which arefabricated as described above, and sealing with the aluminum pouch case.

Comparative Example 2

The secondary battery was fabricated with the same method used inComparative Example 1 except that only the positive electrode activematerial particles D were used.

Performance test

Output Characteristic Test

A test result of measuring resistance through a voltage drop for 10seconds with respect to a pulse current in SOC 50 is represented in afollowing table 1.

TABLE 1 Items Rdis@SOC 50 (mΩ) Example 1 0.91 Example 2 0.92 ComparativeExample 1 0.97 Comparative Example 2 0.92

High-Temperature Service Life Test

A graph illustrating recovery capacity and resistance change afterfinishing 60° C. high-temperature storing of 4.25V fully charged batteryat one week intervals is represented in the FIGURE.

Capacity Retention Rate Test

Capacity retention rate and resistance change rate after 5 weeks ofhigh-temperature storing is represented in Table 2.

TABLE 2 Items Capacity retention rate (%) Resistance change rate (%)Example 1 95.7 18 Example 2 96.0 11 Comparative 95.3 17 Example 1Comparative 91.3 27 Example 2

Capacity retention rate: discharge capacity/1st discharge capacity*100(%) after 5 weeks of storing

Resistance increase rate: R@SOC50/1^(st) R@SOC50 after 5 weeks ofstoring

According to the Examples described above, when the large-grain materialhaving excellent output characteristic and the small-grain materialhaving excellent high-temperature durability are combined, effects canbe obtained in which output characteristic can be enhanced than wheneach material is used alone, while the same performance in thehigh-temperature storing can be obtained as if large-grain materialalone is used.

The present disclosure has been described in detail. However, it shouldbe understood that the detailed description and specific examples, whileindicating preferred embodiments of the disclosure, are given by way ofillustration only, and various changes and modifications within thescope of the disclosure will become apparent to those skilled in the artfrom this detailed description.

What is claimed is:
 1. A mixed positive electrode active material,comprising: a large-grain positive electrode active material having anaverage diameter of 10 μm or greater, and a small-grain positiveelectrode active material having an average diameter of 5 μm or less;and a coating layer disposed on a surface of each of the large-grainpositive electrode active material and the small-grain positiveelectrode active material, wherein the coating layers comprise differentmaterials, wherein one of the materials is a lithium boron oxide-basedcomposition and the other of the materials is a metal oxide.
 2. Themixed positive electrode active material of claim 1, wherein the lithiumboron oxide-based composition includes one of lithium metaborate,lithium triborate, lithium tetraborate, lithium pentaborate, lithiumheptaborate and lithium octaborate, or mixtures of at least two of them.3. The mixed positive electrode active material of claim 1, wherein thelithium boron oxide-based composition includes one of LiBO₂, LiBO₃,LiB₃O₅, Li₂B₃O₇, Li₂B₄O, Li₂B₄O_(7,) Li₂B₅O₈, LiB₇O₁₂, Li₂B₈O₁₃ andLi₁₀B₈O₂₄, or mixtures of at least two of them.
 4. The mixed positiveelectrode active material of claim 1, wherein the lithium boronoxide-based composition is derived from boron (B), a boron (B) compoundor combinations thereof.
 5. The mixed positive electrode active materialof claim 4, wherein the boron (B) compound includes boric acid (H₃BO₃).6. The mixed positive electrode active material of claim 4, wherein thecoating layer including the lithium boron oxide-based composition isformed by mixing the positive electrode active material with the boron(B), the boron (B) compound or combination thereof, and performingthermal treatment.
 7. The mixed positive electrode active material ofclaim 1, wherein the coating layer including the metal oxide is formedby mixing the positive electrode active material with the metal oxide,and performing thermal treatment.
 8. The mixed positive electrode activematerial of claim 1, wherein the lithium in the coating layer includingthe lithium boron oxide-based composition is present in an amount of0.05 to 0.2 parts by weight based on 100 parts by weight of the totalweight of the large-grain positive electrode active material or thesmall-grain positive electrode active material.
 9. The mixed positiveelectrode active material of claim 1, wherein the metal oxide includesone of magnesium oxide, alumina, niobium oxide, titanium oxide andtungsten oxide, or mixtures of at least two of them.
 10. The mixedpositive electrode active material of claim 1, wherein a weight ratiobetween the large-grain positive electrode active material including thecoating layer and the small-grain positive electrode active materialincluding the coating layer is 5:5 to 8:2.
 11. The mixed positiveelectrode active material of claim 1, wherein at least one of thelarge-grain positive electrode active material and the small-grainpositive electrode active material is lithium·nickel·manganese·cobaltcomplex oxide (NMC).
 12. The mixed positive electrode active material ofclaim 11, wherein the lithium·nickel·manganese·cobalt complex oxide isLi(_(1δ))Mn_(x)Ni_(y)Co_((1−x−y−z))M_(z)O₂(M is at least one elementselected from the group consisting of Ti, Zr, Nb, Mo, W, Al, Si, Ga, Geand Sn, and −0.15<δ<0.15, 0.1≤x≤0.5, 0.6<x+y+z<1.0, 0≤z≤0.1)
 13. Apositive electrode, comprising: an electrode current collector; and apositive electrode active material layer formed on at least one surfaceof the electrode current collector and comprising the mixed positiveelectrode active material according to claim 1, a conductor, and abinder.
 14. The positive electrode of claim 13, wherein the conductor isat least one selected from the group consisting of graphite, carbonblack, acetylene black, ketjen black, channel black, furnace black, lampblack, thermal black, carbon fiber, metal fiber, fluorocarbon, aluminumpowder, nickel powder, zinc oxide, potassium titanate and titaniumoxide, and the binder is at least one selected from the group consistingof polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylenecopolymer (PVDF-co-HFP), polyvinyl alcohol, carboxymethyl cellulose(CMC), starch, hydroxypropylcellulose, regenerated cellulose,polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene,polypropylene, ethylene-propylene-diene ter polymer (EPDM), sulfonatedEPDM, styrene butadiene rubber (SBR), fluorine rubber,polyacrylonitrile, and polymethylmethacrylate.
 15. The positiveelectrode of claim 13, wherein the positive electrode is for a secondarybattery.
 16. A method for fabricating a mixed positive electrode activematerial, comprising: (S1) preparing a large-grain positive electrodeactive material having an average diameter of 10 μm or greater and asmall-grain positive electrode active material having an averagediameter of 5 μm or less; (S2) mixing the large-grain positive electrodeactive material or the small-grain positive electrode active materialwith boron (B), a boron (B) compound or combination thereof, andperforming thermal treatment to form a coating layer including a lithiumboron oxide-based composition on a surface of the large-grain positiveelectrode active material or the small-grain positive electrode activematerial; (S3) mixing the positive electrode active material that hasnot gone through the (S2) among the large-grain positive electrodeactive material and the small-grain positive electrode active materialwith a metal oxide, and performing thermal treatment to form a coatinglayer including metal oxide on a surface of the positive electrodeactive material; and (S4) mixing the results of the (S2) and the (S3), aconductor and a binder.
 17. The method for fabricating a mixed positiveelectrode active material of claim 16, wherein the thermal treatment inthe step of forming the coating layer including the lithium boronoxide-based composition is performed between 250 and 500° C., and thethermal treatment in the step of forming the coating layer including themetal oxide is performed between 300 and 800° C.
 18. The method forfabricating a mixed positive electrode active material of claim 16,wherein the thermal treatment is performed under an atmosphericenvironment.
 19. The method for fabricating a mixed positive electrodeactive material of claim 16, wherein the boron (B), the boron (B)compound or combination thereof is present in an amount of 0.05 to 0.2parts by weight based on 100 parts by weight of the total weight of thepositive electrode active material.
 20. The method for fabricating amixed positive electrode active material of claim 16, wherein the metaloxide is present in an amount of 0.05 to 0.2 parts by weight based on100 parts by weight of the total weight of the positive electrode activematerial.